This is a pretty worthwhile way to get a sense of the power of Niagara Falls! The tour takes you to two portals behind Horseshoe Falls and an observation platform next to the falls, for which you will be glad for the disposable raincoat provided (there is also a lower platform, but it's not wheelchair accessible). I knocked off one star because I think it's a bit pricey for what you get; I think if you have to pick between this and the Maid of the Mist, I'd probably go with the Maid of the Mist, but we couldn't talk either of our kids into the idea of the boat tour, so we're glad we did this.
Like many attractions in Niagara, Journey Behind the Falls offers free admission for one person accompanying a person in a wheelchair. Access to the attraction for a wheelchair involves something of a maze of behind-the-scenes ramps and passages, but we didn't have any trouble navigating it, it just seemed kind of long. As far as what you can see, both portals and the upper observation deck are wheelchair accessible; the lower deck (which is the wetter one) is not.